Acetylene is being used more and more as a source material for depositing carbon and carbon-containing films in semiconductor industry. Due to its instability, acetylene is conventionally dissolved in a solvent such as acetone, dimethylformamide (DMF) or N-methylpyrrolidone (NMP) and stored in cylinders containing porous material. When acetylene is withdrawn from the cylinder, an amount of solvent is carried over with the acetylene. The amount of solvent can range, depending on the cylinder temperature and pressure as well as the acetylene flow rate, from about 0.1 percent or less to about 10 percent or greater. The presence of solvent in acetylene is not desirable in applications such as semiconductor applications, especially for the emerging application of acetylene as a source gas for carbon film deposition in the electronic industry. The carryover solvent in acetylene reduces film deposition rate, which affects process uniformity and consistency.
Carbon-based adsorption is widely used in many industries as an effective means to remove volatile organic compounds from gas streams, such as air, nitrogen, etc. Activated carbon is a predominantly amorphous material having an extraordinarily large internal surface area that has been recognized as an effective means for solvent removal and recovery. Solvent removal from acetylene using activated carbon is disclosed, for example, in “Acetylene, Its Properties, Manufacture and Uses” published by Academic Press, New York and London, Vol. 1, page 344, 1965. After adsorption, the spent carbon may be then either disposed of in an approved manner or, more economically, the carbon bed is regenerated for reuse. The carbon bed should be regenerable in a manner so that the purification process can be cost effective.
Due to the catalyst effect of activated carbon, an oxidation reaction of solvent on activated carbon may happen, especially at elevated temperatures, during both adsorption and regeneration processes. Acetylene from an acetylene cylinder contains not only large amount of solvent but also moisture ranging from about 20 parts per million or less to about 100 parts per million or greater due to the presence of water in the solvent added to the acetylene cylinders, residual moisture in the cylinders prior to solvent addition and/or moisture in the acetylene charged to the cylinder. The presence of moisture in acetylene has a markable promoting effect on the oxidation reaction of the solvent on activated carbon. The oxidation reaction can foul the activated carbon so as to reduce its lifetime and also create new impurities in acetylene. For example, the oxidation of acetone may form acetic acid, carbon dioxide, carbon monoxide, and other impurities. Moisture is also an undesirable impurity in acetylene and reduces the efficiency of solvent adsorption on carbon.
The activated carbon not only has the possibility of holding carbon monoxide and carbon dioxide but also contains surface oxides that may produce carbon monoxide and carbon dioxide upon adsorption of a solvent. The carbon monoxide and carbon dioxide will contaminate acetylene when acetylene is passed through the activated carbon bed. Therefore, a need exists for minimizing the oxidation reaction of solvent on carbon and reducing byproduct impurities, for example, carbon dioxide, in acetylene, in order to produce high purity acetylene suitable for use in semiconductor applications. A need also exists for cost effectively regenerating activated carbon for reuse in the purification of acetylene.